You need to contact the Australian Department of Immigration and Citizenship (DIAC) to get evidence of your citizenship. The Department's website says: "You are able to apply for evidence of your Australian citizenship if you ever had, or have been included in, a certificate that says you are an Australian citizen. If you think that you might be an Australian citizen, but you are unsure of your Australian citizenship status, you can also apply." http://www.citizenship.gov.au/current/

The National Archives of Australia cannot give access to a copy of a certificate that was given after 1985.

If you are having problems trying to sort out how to prove you are an Australian citizen, then you need to contact DIAC - if you are in Australia the phone number is 131 880. Otherwise, you can contact an Australian Immigration office outside Australia (maybe there is one close to you). Contact information for Immigration Offices outside Australia can be found at http://www.immi.gov.au/contacts/overseas.